Wood is a naturally occurring polymer composite material. Its utility as a structural element and fuel source is determined by its internal chemistry. Bulk characteristics, such as strength, flammability, and moisture interaction, are governed by the arrangement and reactivity of its constituent biopolymers. Understanding these chemical properties provides insight into how wood performs in various environments.
The Core Chemical Building Blocks
The fundamental structure of wood is built upon three primary biopolymers: cellulose, hemicellulose, and lignin. Cellulose is the most abundant component, typically accounting for 40% to 50% of the wood’s dry weight, and provides the primary structural strength. This molecule is a long, linear chain of linked glucose units, forming a highly ordered, crystalline structure that resists chemical attack.
Hemicellulose is a shorter, more branched polymer, making up about 20% to 35% of the wood mass. Unlike cellulose, it is a mixture of various sugar units like xylose, mannose, and galactose. Its non-uniform, amorphous structure makes it less stable and more susceptible to chemical and thermal breakdown.
Lignin acts as a natural glue, binding the cellulose and hemicellulose fibers together to form the rigid cell wall structure. Comprising 18% to 32% of the wood mass, lignin is a complex, cross-linked polymer built from various phenolic units. This intricate, three-dimensional network imparts compressive strength and stiffness to the wood matrix.
Thermal Reactivity and Combustion Chemistry
The thermal degradation of wood begins with pyrolysis when exposed to sufficient heat. Pyrolysis is the thermal decomposition of wood polymers in the absence or limited presence of oxygen. During this stage, complex molecules break down into simpler chemical products as temperatures rise above 250°C.
The breakdown of cellulose and hemicellulose yields volatile gases, vapors, and a solid carbonaceous residue known as char. These volatile hydrocarbons, which include compounds like carbon monoxide and water vapor, contain a significant portion of the wood’s stored energy. When these gases mix with oxygen and reach their ignition temperature, they undergo rapid oxidation observed as flaming combustion.
The char layer, primarily carbon, is left on the wood surface and acts as a thermal shield, slowing further pyrolysis. If flaming combustion ceases, this char can still react directly with atmospheric oxygen in a slower process called glowing combustion. Char typically accounts for 20% to 30% of the initial dry weight.
Chemical Interaction with Moisture
Wood’s tendency to absorb moisture is a direct consequence of the chemical structure of its main components. Cellulose and hemicellulose contain numerous exposed hydroxyl (-OH) groups, which are chemically polar and strongly attract water molecules. This attraction is satisfied by the formation of strong hydrogen bonds between water molecules and the hydroxyl groups within the cell wall polymers.
This process, known as hygroscopicity, causes water molecules to diffuse into the cell wall, pushing the polymer chains apart. The chemical binding of water leads directly to the physical expansion of the wood, resulting in swelling. Conversely, the loss of this bound water causes the cell wall to shrink, potentially leading to dimensional instability.
Although lignin contains hydroxyl groups, its bulky, cross-linked structure and methoxyl groups reduce the accessibility of these sites to water molecules. This makes lignin less hydrophilic than cellulose or hemicellulose. The extent of water absorption and dimensional change is controlled by the total number of accessible hydroxyl groups within the wood structure.
Natural Chemical Resistance to Degradation
Wood possesses inherent chemical defenses against environmental degradation, largely due to the protective role of lignin. Lignin’s complex, three-dimensional phenolic structure acts as a physical barrier, shielding cellulose and hemicellulose from microbial enzymes. This dense structure makes it difficult for fungi and bacteria to penetrate and break down the cell wall components.
The chemical complexity of lignin also provides resistance to enzymatic degradation, as many enzymes cannot effectively cleave the diverse array of bonds present. However, wood is susceptible to photodegradation when exposed to ultraviolet (UV) light.
UV radiation chemically attacks surface lignin molecules, causing them to break down and form free radicals. This breakdown is responsible for the graying and weathering observed on unprotected wood. The resulting chemical changes reduce the structural integrity of the surface layer and expose the underlying cellulose to further damage.